Wear-Resistant Overlay Welding Test and Technical Application for Coal Chemical Equipment

Literature Overview

This paper by Chen Sunyi from Maoming Gravity Petrochemical Machinery Manufacturing Co., Ltd., published in Petroleum and Chemical Equipment in 2015, addresses the overlay welding challenges encountered during the manufacture of wear-resistant layers for coal chemical equipment. Specifically, the study focuses on a vertical slag collection tank with a lower conical section made of Q345R pressure vessel steel, which requires an internal wear-resistant overlay layer composed of a 309Mo underlay and a Stellite 6 facing layer. The manufacturing process was plagued by cracking defects, and the authors conducted a systematic analysis to identify the root causes and propose effective countermeasures.

Component Description and Overlay Welding Scheme

The component in question is the lower conical section of a vertical slag collection tank used in coal chemical processing. The base material Q345R is a low-carbon pressure vessel steel with a yield strength of approximately 345 MPa, suitable for pressure-containing equipment. The internal surface requires a wear-resistant overlay layer due to the erosive action of slag particles flowing through the tank. The overlay welding scheme consists of a two-layer configuration: a 309Mo underlay layer to ensure metallurgical compatibility between the Q345R base and the high-alloy facing layer, and a Stellite 6 facing layer to provide the primary wear resistance.

Cracking Defect Analysis

The authors conducted a detailed examination of the cracking defects observed during manufacturing. The analysis included evaluation of the number, distribution, and morphology of the cracks. Surface cracking was identified as the primary defect mode, and through comparison with existing research on similar overlay welding applications, the authors confirmed the nature of the cracks as solidification cracks and/or reheat cracks induced by thermal stresses.

The root causes of the surface cracking were attributed to two primary factors. First, uneven thermal stress development during the welding process created localized stress concentrations that exceeded the fracture strength of the weld metal, particularly at the fusion boundary where thermal gradients are steepest. Second, improper welding operation techniques, including excessive welding current, inappropriate travel speed, and inadequate interpass temperature control, contributed to excessive heat input and poor solidification conditions.

Countermeasures and Process Improvements

Based on the defect analysis, the authors proposed several effective countermeasures to improve the overlay welding and heat treatment processes. These included optimizing the welding sequence to ensure uniform thermal distribution across the conical section, controlling welding parameters to reduce heat input per pass, implementing appropriate preheating and interpass temperature control to minimize thermal gradients, and applying post-weld heat treatment to relieve residual stresses. The authors also emphasized the importance of proper welding technique, including maintaining consistent travel speed and avoiding sudden changes in welding direction.

The proposed countermeasures represent a practical application of the PDCA (Plan-Do-Check-Act) methodology: the problem was identified through inspection (Check), root causes were analyzed (Plan), process improvements were implemented (Do), and the results were verified through subsequent welding trials. This systematic approach to defect resolution is a model for engineering practice in overlay welding applications.

Study Insights and Practical Recommendations

This case study provides valuable lessons for engineers working on overlay welding of pressure vessel components. The Q345R base material, while structurally adequate, presents challenges for overlay welding due to its relatively high carbon equivalent and susceptibility to cracking. The two-layer overlay scheme with 309Mo underlay is a well-established approach, but the manufacturing challenges highlight the importance of process discipline and thermal management. For similar applications, engineers should pay particular attention to welding sequence design for conical geometries, as the thermal asymmetry inherent in conical sections can lead to uneven stress development. The emphasis on post-weld heat treatment as a critical step for stress relief is also noteworthy, as residual stresses from overlay welding can be substantial and may lead to delayed cracking or reduced fatigue life if not adequately relieved. This work serves as a practical reminder that overlay welding success depends not only on material selection but also on meticulous process control.